Nature 433, 57–59 (2005)

The x axis of Fig. 1 of this Letter was mislabelled. The correct tick labels (from left to right) should read: ‘Root-free soil 0–10 cm’; ‘Intact soil 0–10 cm’; ‘Root-free soil 20–30 cm’; and ‘Intact soil 20–30 cm’. These errors do not affect any of our conclusions.